At present, the wind rotor of a VAWT has the capability to compete with a horizontal axis three-blade wind rotor on efficiency. However, the wind rotor of the VAWT is hard to be started up without the help of external force. This has brought difficulty to the prevalence of this type of wind turbine.
VAWTs have an advantage over horizontal-axis wind turbines in that they do not need to be orientated into the prevailing wind direction. VAWTs are able to produce a rotational movement irrespective of the wind direction.
The blades can be subject to very high stresses. This is due to the centrifugal forces produced on rotation of the turbine at high rotational speeds. The noise levels can reach unacceptable grades, produced by large vortices shed at the blade tips, rotation of the turbine at high rotational speeds. As the blades alternate between crossing the wind direction and coasting, uneven torque may be produced from the lifting surfaces of VAWTs.
WO2005010355 discloses a VAWT comprising a shaft rotatable about a longitudinal axis and a plurality of substantially rigid blades mechanically coupled to the shaft, each of the plurality of blades comprising an elongate body having an upper end and a lower end, wherein the upper end and the lower end of each blade are rotationally off-set from each other about the longitudinal axis such that each blade has a helix-like form, the section of the elongate body of each blade, taken perpendicularly to the longitudinal axis, being shaped as an aerofoil having a leading edge and a trailing edge and a camber line defined between the leading edge and the trailing edge, characterised in that the aerofoil is accurately shaped such that the camber line lies along a line of constant curvature having a finite radius of curvature.
WO2009151359 discloses a wind turbine with vertical axis and having a plurality of axial extending turbine blades where the structure of each blade varies along the axial extension of the blade.